Oxypropylene polymers having a reactive silicon group(s) are polymers which are capable of becoming liquid polymers and are cured in the presence of moisture at room temperature to form rubbery elastic cured articles. Accordingly, these polymers are used in the fields of elastic sealants and adhesives for architectural structures.
It is desirable that these polymers have an appropriate viscosity when blended or applied. Further, it is desirable that these polymers have a given molecular weight to provide cured articles having desired mechanical characteristics, particularly rubber elasticity high in flexibility.
Many methods for preparing organic polymers having reactive silicon groups in the molecule have been proposed. For example, organic polymers (trade mark: MS polymer) having a main chain composed of an oxypropylene polymer and methoxysilyl groups at terminals are manufactured by Kanegafuchi Kagaku Kogyo K.K. and are commercially available.
However, there is conventionally a difficulty in manufacturing oxypropylene polymers having a narrow molecular weight distribution (Mw/Mn ratio measured by GPC is small) and a high molecular weight. Accordingly, only polymers having a wide molecular weight distribution (Mw/Mn measured by GPC is large) including oxypropylene polymers having reactive silicon groups have been conventionally used.
It has recently been reported that an oxypropylene polymer having a narrow molecular weight distribution has been obtained. Polymers having a main chain composed of an oxypropylene polymer having a narrow molecular weight distribution and reactive silicon groups, as terminal groups, introduced into the terminals of the polymer chain, have a low viscosity before curing.
The present inventors have found that a composition comprising a mixture of a linear polymer having no branched chain and a polymer having at least one branched chain in a curable composition comprising oxypropylene polymers having a narrow molecular weight distribution gives a cured article having a greatly improved tear strength which is unexpectedly higher than that which can be estimated from tear strength of cured articles of each the polymers. The present invention has been accomplished on the basis of this finding.